Underwater photography and videography typically employ white light illumination sources. Wavelength-dependent losses occur as the light propagates through water. Longer wavelengths (e.g., red light) attenuate more rapidly than shorter wavelengths (e.g., blue light). Consequently, images are not captured in true color. Moreover, as the propagation distance changes, the changes in loss at different wavelengths are not equal. Absorption is the primary cause for loss in underwater propagation. Absorption is exponential with respect to the propagation distance, where the exponential value varies according to the wavelength of the light. Other factors such as scattering affect loss but are substantially independent of the wavelength of the visible light.
One technique known in the art to reduce the effect of wavelength-dependent loss on image color is based on applying a colored filter to the underwater camera. The filter attenuates blue light to substantially compensate for the dominant loss of red light in underwater propagation. As the loss is distance-dependent, a different filter is needed for different ranges to the object or scene of interest. This technique is impractical for many reasons. The distance to the scene may not be accurately controllable or may change rapidly in time. In some instances, there may not be sufficient time to change the filter or the camera may be remotely located so that the filter cannot be changed.
Another technique known in the art is based on determining the range to the object or scene to be imaged. According to this passive imaging technique, color absorption correction is applied to image data acquired by the camera. The camera must be modified or a separate device must be provided to perform the correction processing of the acquired image data.
What is needed is a method to adaptively illuminate an underwater object or scene to achieve true color images in real-time. The present invention satisfies this need and provides additional advantages.